Light-emitting semiconductor device using gallium nitride group compound

ABSTRACT

Disclosed herein are (1) a light-emitting semiconductor device that uses a gallium nitride compound semiconductor (Al x Ga 1−x N) in which the n-layer of n-type gallium nitride compound semiconductor (Al x Ga 1−x N) is of double-layer structure including an n-layer of low carrier concentration and an n + -layer of high carrier concentration, the former being adjacent to the i-layer of insulating gallium nitride compound semiconductor (Al x Ga 1−x N); (2) a light-emitting semiconductor device of similar structure as above in which the i-layer is of double-layer structure including an i L -layer of low impurity concentration containing p-type impurities in comparatively low concentration and an i H -layer of high impurity concentration containing p-type impurities in comparatively high concentration, the former being adjacent to the n-layer; (3) a light-emitting semiconductor device having both of the above-mentioned features and (4) a method of producing a layer of an n-type gallium nitride compound semiconductor (Al x Ga 1−x N) having a controlled conductivity from an organometallic compound by vapor phase epitaxy, by feeding a silicon-containing gas and other raw material gases together at a controlled mixing ratio.

This is a division of application Ser. No. 08/956,950, filed Oct. 23,1997 now U.S. Pat. No. 6,249,012; which is a divisional of Ser. No.08/556,232 filed Nov. 9, 1995, now U.S. Pat. No. 5,733,796; which is acontinuation of Ser. No. 08/179,242, filed Jan. 10, 1994; now abandoned;which is a divisional of Ser. No. 07/926,022, filed Aug. 7, 1992, nowU.S. Pat. No. 5,278,433; which is a continuation of Ser. No. 07/661,304,filed Feb. 27, 1991; now abandoned, the contents of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting semiconductor deviceusing gallium nitride group compound which emits a blue light.

2. Description of the Prior Art

It is known that GaN compound semiconductor can be made into alight-emitting semiconductor device, such as a light-emitting diode(LED), which emits a blue light. The GaN compound semiconductor attractsattention because of its high light-emitting efficiency resulting fromdirect transition and of its ability to emit a blue light which is oneof three primary colors.

The light-emitting diode manufactured from the GaN compoundsemiconductor is composed of an n-layer and an i-layer grown thereon.The n-layer of the GaN compound semiconductor with n-type conduction isdirectly grown on a surface of a sapphire substrate or grown on a bufferlayer of aluminum nitride formed on the substrate. The i-layer ofinsulating (i-type) GaN compound semiconductor doped with p-typeimpurities is grown on the n-layer. (See Japanese Patent Laid-open Nos.119196/1987 and 188977/1988.) The light-emitting diode of this structurehas room for improvement in luminous intensity. In addition, itcomprises no p-n junction but it is made by joining the i-layer andn-layer.

An electric property of the GaN compound semiconductor shows inherentlyn-type conduction even though it is not deliberately doped with n-typeimpurities, and unlike silicon and similar semiconductors, when it isdoped with zinc of p-type impurities, the electric property shows notp-type conduction but insulation. Moreover, the production of n-type GaNinvolves many difficulties in controlling conductivity.

SUMMARY OF THE INVENTION

It is the first object of the present invention to improve a luminousefficiency of a GaN group light-emitting diode.

It Is the second object of the present invention to provide a new layerstructure which improves a luminous efficiency of a GaN group lightemitting diode.

It is the third object of the present invention to provide a technologyfor production of n-type GaN group compound semiconductor in whichconductivity is easily controlled.

After experience in the manufacture of the above-mentioned GaNlight-emitting diode, the present inventors established a technology fora vapor phase epitaxy of the GaN group semiconductor with organometalcompound. This technology enables a production of a gas-phase grown GaNlayer of high purity. In other words, this technology provides n-typeGaN with high resistivity without doping with impurities, unlike theconventional technology which provides n-type GaN with low resistivitywhen no doping is performed.

The First Feature of the Invention

The first feature of the present invention resides in a light-emittingsemiconductor device composed of an n-layer of n-type gallium nitridegroup compound semiconductor (Al_(x)Ga_(1−x)N; inclusive of x=0) and ani-layer of insulating (i-type) gallium nitride compound semiconductor(Al_(X)Ga_(1−x)N; inclusive of x=0) doped with p-type impurities, inwhich the n-layer is of double-layer structure including an n-layer oflow carrier concentration and an n⁺-layer of high carrier concentration,the former being adjacent to the i-layer.

According to the present invention, the n-layer of low carrierconcentration should preferably have a carrier concentration of1×10¹⁴/cm³ to 1×10¹⁷/cm³ and have a thickness of 0.5 to 2 μm. In casethat the carrier concentration is higher than 1×10¹⁷/cm³, the luminousintensity of the light-emitting diode decreases. In case that thecarrier concentration is lower than 1×10¹⁴/cm³, since the seriesresistance of the light-emitting diode increases, an amount of heatgenerated in the n-layer increases when a constant current is suppliedto it. In case that the layer thickness is greater than 2 μm, since theseries resistance of the light-emitting diode increases, the amount ofheat generated in the n-layer increases when the constant current issupplied to it. In case that the layer thickness is smaller than 0.5 μm,the luminous intensity of the light-emitting diode decreases.

In addition, the n⁺-layer of high carrier concentration shouldpreferably contain a carrier concentration of 1×10¹⁷/cm³ to 1×10¹⁹/cm³and have a thickness of 2-10 μm. In case that the carrier concentrationis higher than 1×10¹⁹/cm³, the n⁺-layer is poor in crystallinity. Incase that the carrier concentration is lower than 1×10¹⁷/cm³, since theseries resistance of the light-emitting diode increases, an amount ofheat generated in the n⁺-layer increases when a constant current issupplied to it. In case that the layer thickness is greater than 10 μm,the substrate of the light-emitting diode warps. In case that the layerthickness is smaller than 2 μm, since the series resistance of thelight-emitting diode increases, the amount of heat generated in then⁺-layer increases when the constant current is supplied to it.

In the first feature of the present invention, it is possible toincrease an intensity of blue light emitted from the light-emittingdiode by making the n-layer in double-layer structure including ann-layer of low carrier concentration and an n⁺-layer of high carrierconcentration, the former being adjacent to the i-layer. In other words,the n-layer as a whole has a low electric resistance owing to then⁺-layer of high carrier concentration, and hence the light-emittingdiode has low series resistance and generates less heat when a constantcurrent is supplied to it. The n-layer adjacent to the i-layer has alower carrier concentration or higher purity so that it contains asmaller amount of impurity atoms which are deleterious to the emissionof blue light from the light-emission region (i-layer and its vicinity).Due to the above-mentioned functions, the light-emitting diode of thepresent invention emits a blue light of higher intensity.

The Second Feature of the Invention

The second feature of the present invention resides in a light-emittingsemiconductor device composed of an n-layer of n-type gallium nitridecompound semiconductor (Al_(x)Ga_(1−x)N; inclusive of x=0) and ani-layer of i-type gallium nitride compound semiconductor(Al_(x)Ga_(1−x)N; inclusive of x=0) doped with p-type impurities, inwhich the i-layer is of double-layer structure including an i_(L)-layercontaining p-type impurities in comparatively low concentration and ani_(H)-layer containing p-type impurities in comparatively highconcentration, the former being adjacent to the n-layer.

According to the present invention, the i_(L)-layer of low impurityconcentration should preferably contain the impurities in concentrationof 1×10¹⁶/cm³ to 5×10¹⁹/cm³ and have a thickness of 0.01 to 1 μm. Incase that impurity concentration is higher than 5×10¹⁹/cm³, since theseries resistance of the light-emitting diode increases, an initialvoltage to start emitting light at increases. In case that the impurityconcentration is lower than 1×10¹⁶/cm³, the semiconductor of thei_(L)-layer shows n-type conduction. In case that the layer thickness isgreater than 1 μm, since the series resistance of the light-emittingdiode increases, the initial voltage to start emitting light atincreases. In case that the layer thickness is smaller than 0.01 μm, thelight-emitting diode has the same structure as that of the conventionalone.

In addition, the i_(H)-layer of high impurity concentration shouldpreferably contain the impurities in concentration of 1×10¹⁹/cm³ to5×10²⁰/cm³ and have a thickness of 0.02 to 0.3 μm. In case that theimpurity concentration is higher than 5×10²⁰/cm³, the semiconductor ofthe i_(H)-layer is poor in crystallinity. In case that the impurityconcentration is lower than 1×10¹⁹/cm³, the luminous intensity of thelight-emitting diode decreases. In case that the layer thickness isgreater than 0.3 μm, since the series resistance of the light-emittingdiode increases, an initial voltage to start emitting light atincreases. In case that the layer thickness is smaller than 0.02 μm, thei-layer is subject to breakage.

In the second feature of the present invention, it is possible toincrease an intensity of blue light emitted from the light-emittingdiode by making the i-layer in double-layer structure including ani_(L)-layer containing p-type impurities in comparatively lowconcentration and an i_(H)-layer containing p-type impurities incomparatively high concentration, the former being adjacent to then-layer. In other words, this structure (in which the i-layer adjacentto the n-layer is the i_(L)-layer of low impurity concentration) enableselectrons to be injected from the n-layer into the i_(H)-layer of highimpurity concentration without being trapped in the i_(L)-layer and itsvicinity. Therefore, this structure enables electrons to pass throughthe i_(L)-layer of low impurity concentration, which is poor in luminousefficacy, adjacent to the n-layer, and to reach the i_(H)-layer of highimpurity concentration in which electrons emit light with a highefficiency.

The Third Feature of the Invention

The third feature of the present invention resides in a light-emittingsemiconductor device composed of an n-layer of n-type gallium nitridecompound semiconductor (Al_(x)Ga_(1−x)N; inclusive of x=0) and ani-layer of i-type gallium nitride compound semiconductor(Al_(x)Ga_(1−x)N; inclusive of x=0) doped with p-type impurities, inwhich the n-layer is of double-layer structure including an n-layer oflow carrier concentration and an n⁺-layer of high carrier concentration,the former being adjacent to the i-layer, and the i-layer is ofdouble-layer structure including an i_(L)-layer containing p-typeimpurities in comparatively low concentration and an i_(H)-layercontaining p-type impurities in comparatively high concentration, theformer being adjacent to the n-layer.

The third feature of the present invention is a combination of the firstfeature (the n-layer of double layer structure) and the second feature(the i-layer of double layer structure). Therefore, the n-layer of lowcarrier concentration, the n⁺-layer of high carrier concentration, thei_(L)-layer of low impurity concentration, and the i_(H)-layer of highimpurity concentration should correspond to the respective layers as thefirst and second features. The carrier concentration and layer thicknessare defined in the same manner as in the first and second features.

In the third feature of the present invention, it is possible toincrease an intensity of blue light from the light-emitting diode bymaking the n-layer in double-layer structure including an n-layer of lowcarrier concentration and an n⁺-layer of high carrier concentration, theformer being adjacent to the i-layer, and also by making the i-layer indouble-layer structure including an i_(L)-layer containing p-typeimpurities in comparatively low concentration and an i_(H)-layercontaining p-type impurities in comparatively high concentration, theformer being adjacent to the n-layer.

In other words, the n-layer as a whole has a low electric resistanceowing to the n⁺-layer of high carrier concentration, which makes itpossible to apply an effective voltage to the junction between thei_(L)-layer and n-layer of low carrier concentration. Having a lowcarrier concentration, the n-layer adjacent to the i_(L)-layer does notpermit non-light-emitting impurity atoms to diffuse into thei_(L)-layer. In addition, this structure (in which the i-layer adjacentto the n-layer is the i_(L)-layer of low impurity concentration) permitselectrons to be injected from the n-layer into the i_(H)-layer of highimpurity concentration without being trapped in the i_(L)-layer.Therefore, this structure permits electrons to pass through thei_(L)-layer of low impurity concentration, which is poor in luminousefficacy, adjacent to the n-layer, and to reach the i_(H)-layer of highimpurity concentration in which electrons emit light with a highefficiency.

For this reason, the light-emitting diode of the present invention has amuch higher luminous efficacy than the one having the conventionalsimple i-n junction.

The Fourth Feature of the Invention

The fourth feature of the present invention resides in a method ofproducing an n-type gallium nitride compound semiconductor(Al_(x)Ga_(1−x)N; inclusive of x=0) from organometal compound by vaporphase epitaxy. This method comprises a step of feeding asilicon-containing gas and other raw material gases together at a propermixing ratio so that the conductivity of the compound semiconductor isdesirably controlled. The mixing ratio is adjusted such that siliconenters the layer of gallium nitride compound semiconductor grown byvapor phase epitaxy and functions as the donor therein. Thus it ispossible to vary the conductivity of the n-type layer by adjusting themixing ratio.

The Fifth Feature of the Invention

The fifth feature of the present invention resides in a method forproducing a light-emitting semiconductor device. The method comprisestwo steps. The first step involves growing an n⁺-layer of high carrierconcentration (which is an n-type gallium nitride compound semiconductor(Al_(x)Ga_(1−x)N; inclusive of x=0) having a comparatively highconductivity) by vapor phase epitaxy from organometal compound. Thevapor phase epitaxy is accomplished on a sapphire substrate having abuffer layer of aluminum nitride by feeding a silicon-containing gas andother raw material gases together at a proper mixing ratio. The secondstep involves growing an n-layer of low carrier concentration (which isan n-type gallium nitride compound semiconductor (Al_(x)Ga_(1−x)N;inclusive of x=0) having a comparatively low conductivity) by vaporphase epitaxy from organometal compound. The vapor phase epitaxy isaccomplished on the n⁺-layer formed by the first step by feeding rawmaterial gases excluding the silicon-containing gas. The n-layer ofdouble-layer structure can be produced by properly controlling themixing ratio of a silicon-containing gas and other raw material gases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a structure of a light-emitting diode shownas Example 1 of the present invention.

FIGS. 2 to 7 are sectional views illustrating processes for producing alight-emitting diode shown as to Example 1 of the present invention.

FIG. 8 is a diagram showing relationship between a carrier concentrationof an n-layer of low carrier concentration and intensity or wavelengthof emitted light with respect to a light-emitting diode shown as Example1 of the present invention.

FIG. 9 is a diagram showing a structure of a light-emitting diode shownas Example 2 of the present invention.

FIGS. 10 to 15 are sectional views illustrating processes for producinga light-emitting diode shown as Example 2 of the present invention.

FIG. 16 is a diagram showing relationship between an impurityconcentration of an i_(H)-layer of high impurity concentration andintensity or wavelength of emitted light with respect to alight-emitting diode shown as Example 2 of the present invention.

FIG. 17 is a diagram showing a structure of a light emitting diode shownas Example 3 of the present invention.

FIGS. 18 to 23 are sectional views illustrating processes for producinga light-emitting diode shown as Example 3 of the present invention.

FIG. 24 is a diagram showing relationship between a carrierconcentration of an n-layer of low carrier concentration and intensityor wavelength of emitted light with respect to a light-emitting diodeshown as Example 3 of the present invention.

FIG. 25 is a diagram showing relationship between an impurityconcentration of an i_(H)-layer of high impurity concentration andintensity or wavelength of emitted light with respect to alight-emitting diode shown as Example 3 of the present invention.

FIG. 26 is a diagram showing the relationship between a flow rate ofsilane gas and electrical properties of an n-layer formed by vapor phaseepitaxy in a process shown as Example 4 of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in more detail with reference to thefollowing examples.

EXAMPLE 1

In FIG. 1 there is shown a light-emitting diode 10 which has a sapphiresubstrate 1 on which is formed a buffer layer of 500 Å thick AlN. On thebuffer layer 2 are consecutively formed an n⁺-layer 3 of high carrierconcentration of 2.2 μm thick GaN and an n-layer 4 of low carrierconcentration of 1.5 μm thick GaN. And an i-(insulating) layer 6 of 0.2μm thick GaN is formed on the n-layer 4. Aluminum electrodes 7 and 8 areconnected to the i-layer 6 and n⁺-layer 3, respectively.

This light-emitting diode 10 was produced by metalorganic vapor phaseepitaxy in the following manner. (This process is referred to as MOVPEhereinafter.)

The gases employed in this process are NH₃, H₂ (as carrier gas),trimethyl gallium (Ga(CH₃)₃) (TMG hereinafter), trimethyl aluminum(Al(CH₃)₃) (TMA hereinafter), silane (SiH₄), and diethyl zinc (DEZhereinafter).

The sapphire substrate 1 of single crystal, with its principal crystalplane (a-surface {11{overscore (2)}0}) cleaned by solvent washing andheat treatment, was set on the susceptor placed in a reaction chamber ofan MOVPE apparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100° C. withH₂ flowing through the reaction chamber at a flow rate of 2 l/min undernormal pressure.

On the sapphire substrate 1 was formed the AlN buffer layer 2 (about 500Å thick) at 400° C. by supplying H₂ at a flow rate of 20 l/min, NH₃ at aflow rate of 10 l/min, and TMA at a flow rate of 1.8×10⁻⁵ mol/min.

On the buffer layer 2 was formed the n⁺-layer 3 of high carrierconcentration (1.5×10¹⁸/cm³) of 2.2 μm thick GaN by supplying H₂ at aflow rate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flowrate of 1.7×10⁻⁴ mol/min, and silane (diluted to 0.86 ppm with H₂) at aflow rate of 200 ml/min, with the sapphire substrate 1 kept at 1150° C.

On the n⁺-layer 3 was formed the n-layer 4 of low carrier concentration(1×10¹⁵/cm³) of 1.5 μm thick GaN by supplying H₂ at a flow rate of 20l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate of1.7×10⁻⁴ mol/min, with the sapphire substrate 1 kept at 1150° C.

On the n-layer 4 was formed the i-layer 6 of 0.2 μm thick GaN bysupplying H₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min,TMG at a flow rate of 1.7×10⁻⁴ mol/min, and DEZ at a flow rate of1.5×10⁻⁴ mol/min, with the sapphire substrate 1 kept at 900° C.

Thus there was obtained the multi-layer structure as shown in FIG. 2.

On the i-layer 6 was formed a 2000 Å thick SiO₂ layer 11 by sputteringas shown in FIG. 3. On the SiO₂ layer 11 was formed a photoresist layer12 which subsequently underwent a photolithographic processing to make apattern corresponding to a figure of the electrode connected to then⁺-layer 3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂layer 11 underwent etching with hydrofluoric acid for its removal, asshown in FIG. 4.

The exposed part (not covered by the photoresist layer 12 and the SiO₂layer 11) of the i-layer 6 underwent dry etching with CCl₂F₂ gas at aflow rate of 10 cc/min and a high-frequency electric power of 0.44 W/cm²in a vacuum of 0.04 Torr and subsequently underwent dry etching withargon. The dry etching removed not only the exposed part of the i-layer6 but also the n-layer 4 and the upper part of the n⁺-layer 3 which areunderneath the exposed part of the i-layer 6, as shown in FIG. 5.

The SiO₂ layer 11 remaining on the i-layer 6 was removed withhydrofluoric acid as shown in FIG. 6.

On the entire surface of the sample was formed an Al layer 13 by vapordeposition as shown in FIG. 7. On the Al layer 13 was formed aphotoresist layer 14 which subsequently underwent the photolithographicprocessing to make a pattern corresponding to a figure of the electrodesconnected to the n⁺-layer 3 and the i-layer 6, respectively.

The exposed part (not covered by the photoresist layer 14) of the Allayer 13 underwent etching with nitric acid as shown in FIG. 7. Thephotoresist 14 was removed with acetone. Thus there were formed theelectrode 8 for the n⁺-layer 3 and the electrode 7 for the i-layer 6.

Such an above-mentioned process could make a gallium nitridelight-emitting element of MIS (metal-insulator-semiconductor) structureas shown in FIG. 1.

The thus obtained light-emitting diode 10 was found to have a luminousintensity of 0.2 mcd. This value is 4 times higher than that of theconventional light-emitting diode which is composed simply of an i-layerwith impurity concentration of 2×10²⁰/cm³ and a 4 μm thick n-layer withcarrier concentration of 5×10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that thenumber of luminescent points is much greater than that of theconventional light-emitting diode.

Several samples were prepared in the same manner as mentioned aboveexcept that the carrier concentration in the n-layer of low carrierconcentration was varied, and they were tested for luminous intensityand emission spectrum. The results are shown in FIG. 8. It is noted thatthe luminous intensity decreases and the emission spectrum shifts to thered side according as the carrier concentration increases. This effectis estimated to be caused by that atoms of silicon as doping atomsdiffuse or mix into the i-layer 6 as impurity atoms.

EXAMPLE 2

In FIG. 9 there is shown a light-emitting diode 10 which has a sapphiresubstrate 1 on which is formed a 500 Å thick AlN buffer layer 2. On thebuffer layer 2 are consecutively formed a 4 μm thick GaN n-layer 3 withcarrier concentration of 5×10¹⁷/cm³, an i_(L)-layer 5 of low impurityconcentration of 5×10¹⁹/cm³ of Zn, and an i_(H)-layer 6 of high impurityconcentration (2×10²⁰/cm³ of Zn). To the i_(H)-layer 6 and n-layer 3 areconnected aluminum electrodes 7 and 8, respectively.

This light-emitting diode 10 was produced by the MOVPE.

The gases employed in this process are NH₃, H₂ (as carrier gas),trimethyl gallium TMG, trimethyl aluminum TMA, and diethyl zinc DEZ.

The sapphire substrate 1 of single crystal, with its principal crystalplane (c-surface {0001}) cleaned by solvent washing and heat treatment,was set on the susceptor placed in the reaction chamber of the MOVPEapparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100° C. withH₂ flowing through the reaction chamber at a flow rate of 2 l/min undernormal pressure.

On the sapphire substrate 1 was formed the AIN buffer layer 2 (about 500Å thick) at 400° C. by supplying H₂ at a flow rate of 20 l/min, NH₃ at aflow rate of 10 l/min, and TMA at a flow rate of 1.8×10⁻⁵ mol/min.

On the buffer layer 2 was formed the 4 μm thick GaN n-layer 3 withcarrier concentration of 1.5×10¹⁷/cm³ by supplying H₂ at a flow rate of20 l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate of1.7×10⁻⁴ mol/min with stopping the feeding of TMA, with the sapphiresubstrate 1 kept at 1150° C.

On the n-layer 3 was formed the 0.2 μm thick GaN i_(L)-layer 5 of lowimpurity concentration (5×10¹⁹/cm³ of Zn) by supplying H₂ at a flow rateof 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of1.7×10⁻⁴ mol/min, and DEZ at a flow rate of 1.5×10⁻⁴ mol/min, with thesapphire substrate 1 kept at 1000° C.

On the i_(L)-layer 5 was formed the 0.2 μm thick GaN i_(H)-layer 6 ofhigh impurity concentration (2×10²⁰/cm³ of Zn) by supplying H₂ at a flowrate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of1.7×10⁻⁴ mol/min, and DEZ at a flow rate of 1.5×10⁻⁴ mol/min, with thesapphire substrate 1 kept at 900° C.

Thus there was obtained the multi-layer structure as shown in FIG. 10.

On the i_(H)-layer 6 was formed the 2000 Å thick SiO₂ layer 11 bysputtering as shown in FIG. 11. On the SiO₂ layer 11 was formed aphotoresist layer 12 which subsequently underwent the photolithographicprocessing to make a pattern corresponding to the figure of theelectrode connected to the n-layer 3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂layer 11 underwent etching with hydrofluoric acid for its removal, asshown in FIG. 12.

The exposed part (not covered by the photoresist layer 12 and the SiO₂layer 11) of the i_(H)-layer 6 underwent dry etching with CCl₂F₂ gas ata flow rate of 10 cc/min and a high-frequency electric power of 0.44W/cm2 in a vacuum of 0.04 Torr and subsequently underwent dry etchingwith argon. The dry etching removed not only the exposed part of thei_(H)-layer 6 but also the i_(L)-layer 5 and the upper part of, then-layer 3 which are underneath the exposed part of the i_(H)-layer 6, asshown in FIG. 13.

The SiO₂ layer 11 remaining on the i_(H)-layer 6 was removed withhydrofluoric arid as shown in FIG. 14.

On the entire surface of the sample was formed an Al layer 13 by vapordeposition as shown in FIG. 15. On the Al layer 13 was formed thephotoresist layer 14 which subsequently underwent the photolithographicprocessing to make a pattern corresponding to the figure of theelectrodes connected to the n-layer 3 and the i_(H)-layer 6,respectively.

The exposed part (not covered by the photoresist layer 14) of the Allayer 13 underwent etching with nitric acid as shown in FIG. 15. Thephotoresist 14 was removed with acetone. Thus there were formed theelectrode 8 for the n-layer 3 and the electrode 7 for the i_(H)-layer 6.

Such an above-mentioned process could make a gallium nitridelight-emitting element of MIS structure as shown in FIG. 9.

The thus obtained light-emitting diode 10 was found to have a luminousintensity of 0.2 mcd. This value is 4 times higher than that of theconventional light-emitting diode which is composed simply of a 0.2 μmthick i-layer with impurity concentration of 2×10²⁰/cm³ and a 4 μm thickn-layer with carrier concentration of 5×10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that thenumber of luminescent points is much greater than that of theconventional light-emitting diode.

Several samples were prepared in the same manner as mentioned aboveexcept that the impurity concentration in the i_(H)-layer 6 of highimpurity concentration was varied, and they were tested for luminousintensity and emission spectrum. The results are shown in FIG. 16. It isnoted that the luminous intensity has a peak value and the emissionspectrum shifts to a longer wavelength side when the impurityconcentration increases.

EXAMPLE 3

In FIG. 17 there is shown a light-emitting diode 10 which has a sapphiresubstrate 1 on which is formed a 500 Å thick AlN buffer layer 2. On thebuffer layer 2 are consecutively formed a 2.2 μm thick GaN n⁺-layer 3 ofhigh carrier concentration (1.5×10¹⁸/cm³), a 1.5 μm thick GaN n-layer 4of low carrier concentration (1×10¹⁵/cm³), an i_(L)-layer 5 of lowimpurity concentration (5×10¹⁹/cm³ of Zn), and an i_(H)-layer 6 of highimpurity concentration (2×10²⁰/cm³ of Zn). To the i_(H) layer 6 andn⁺-layer 3 are connected aluminum electrodes 7 and 8, respectively. Thislight-emitting diode 10 was produced by the MOVPE with organometalcompound in the following manner.

The gases employed in this process are NH₃, H₂ (as carrier gas),trimethyl gallium (Ga(CH₃)₃) (TMG), trimethyl aluminum (Al(CH₃)₃) (TMA),silane (SiH₄), and diethyl zinc (DEZ).

The sapphire substrate 1 of single crystal, with, its principal crystalplane ( c-surface {0001}) cleaned by solvent washing and heat treatment,was set on the susceptor placed in the reaction chamber of the MOVPEapparatus.

The sapphire substrate 1 underwent vapor phase etching at 1100° C. withH₂ flowing through the reaction chamber at a flow rate of 2 l/min undernormal pressure.

On the sapphire substrate 1 was formed the AlN buffer layer 2 (about 500Å thick) at 400° C. by supplying H₂ at a flow rate of 20 l/min, NH₃ at aflow rate of 10 l/min, and TMA at a flow rate of 1.8×10⁻⁵ mol/min.

On the buffer layer 2 was formed the 2.2 μm thick GaN n⁺-layer 3 of highcarrier concentration (1.5×10¹⁸/cm³) by supplying H₂ at a flow rate of20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of 1.7×10⁻⁴mol/min, and silane (diluted to 0.86 ppm with H₂) at a flow rate of 200ml/min for 30 minutes, with the sapphire substrate 1 kept at 1150° C.

On the n⁺-layer 3 was formed the 1.5 μm thick GaN n-layer 4 of lowcarrier concentration (1×10¹⁵/cm³) by supplying H₂ at a flow rate of 20l/min, NH₃ at a flow rate of 10 l/min, and TMG at a flow rate of1.7×10⁻⁴ mol/min, with the sapphire substrate 1 kept at 1150° C.

On the n-layer 4 was formed the 0.2 μm thick GaN i_(L)-layer 5 of lowimpurity concentration (5×10¹⁹/cm³ of Zn) by supplying H₂ at a flow rateof 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of1.7×10⁻⁴ mol/min, and DEZ at a flow rate of 1.5×10⁻⁴ mol/min, with thesapphire substrate 1 kept at 1000° C.

On the i_(L)-layer 5 was formed the 0.2 μm thick GaN i_(H)-layer 6 ofhigh impurity concentration (2×10²⁰/cm³ of Zn) by supplying H₂ at a flowrate of 20 l/min, NH₃ at a flow rate of 10 l/min, TMG at a flow rate of1.7×10⁻⁴ mol/min, and DEZ at a flow rate of 1.5×10⁻⁴ mol/min, with thesapphire substrate 1 kept at 900° C.

Thus there was obtained the multi-layer structure as shown in FIG. 18.

On the i_(H)-layer 6 was formed the 2000 Å thick SiO₂ layer 11 bysputtering as shown in FIG. 19. On the SiO₂ layer 11 was formed aphotoresist layer 12 which subsequently underwent the photolithographicprocessing to make a pattern for the electrode connected to the n⁺-layer3.

The exposed part (not covered by the photoresist layer 12) of the SiO₂layer 11 underwent etching with hydrofluoric acid for its removal, asshown in FIG. 20.

The exposed part (not covered by the photoresist layer 12 and the SiO₂layer 11) of the i_(H)-layer 6 underwent dry etching with CCl₂F₂ gas ata flow rate of 10 cc/min and a high-frequency electric power of 0.44W/cm² in a vacuum of 0.04 Torr and subsequently underwent dry etchingwith argon. The dry etching removed not only the exposed part of thei_(H)-layer 6 but also the i_(L)-layer 5 and the n-layer 4 and the upperpart of the n⁺-layer 3 which are underneath the exposed part of thei_(H)-layer 6, as shown in FIG. 21.

The SiO₂ layer 11 remaining on the i_(H)-layer 6 was removed withhydrofluoric arid as shown in FIG. 22.

On the entire surface of the sample was formed an Al layer 13 by vapordeposition as shown in FIG. 23. On the Al layer 13 was formed thephotoresist layer 14 which subsequently underwent the photolithographicprocessing to make a pattern for the electrodes connected to then⁺-layer 3 and the i_(H)-layer 6, respectively.

The exposed part (not covered by the photoresist layer 14) of the Allayer 13 underwent etching with nitric acid as shown in FIG. 23. Thephotoresist 14 was removed with acetone. Thus there were formed theelectrode 8 for the n⁺-layer 3 and the electrode 7 for the i_(H)-layer6.

Such an above-mentioned process could make a gallium nitridelight-emitting element of MIS structure as shown in FIG. 17.

The thus obtained light-emitting diode 10 was found to have a luminousintensity of 0.4 mcd. This value is 8 times higher than that of theconventional light-emitting diode which is composed simply of a 0.2 μmthick i-layer with impurity concentration of 2×10²⁰/cm³ and a 4 μm thickn-layer with a carrier concentration of 5×10¹⁷/cm³.

In addition, the inspection of the luminescent surface revealed that thenumber of luminescent points is much greater than that of theconventional light-emitting diode.

Several samples were prepared in the same manner as mentioned aboveexcept that the carrier concentration in the n-layer 4 of low carrierconcentration was varied, and they were tested for luminous intensityand emission spectrum. The results are shown in FIG. 24. It is notedthat the luminous intensity decreases and the emission spectrum shiftsto the red side according as the carrier concentration increases.

Also, several samples were prepared in the same manner as mentionedabove except that the impurity concentration in the i_(H)-layer 6 ofhigh impurity concentration was varied, and they were tested forluminous intensity and emission spectrum. The results are shown in FIG.25. It is noted that the luminous intensity has a peak value and theemission spectrum shifts to a longer wavelength side when the impurityconcentration increases.

EXAMPLE 4

A light-emitting diode 10 of the same structure as in Example 1 wasprepared in the same manner as in Example 1 according to the steps shownin FIGS. 2 to 7. The resistivity of the n⁺-layer 3 was varied in therange of 3×10⁻¹ Ωcm to 8×10⁻³ Ωcm by changing the conditions of thevapor phase epitaxy for the n⁺-layer 3 of high carrier concentration, asshown in FIG. 26. The vapor phase epitaxy was carried out by supplyingH₂ at a flow rate of 20 l/min, NH₃ at a flow rate of 10 l/min,TMG-carrying H₂ at a flow rate of 100 cc/min, and H₂-diluted silane(0.86 ppm) at a flow rate of 10 cc/min to 300 cc/min. (The TMG-carryingH₂ was prepared by bubbling H₂ in TMG cooled at −15° C.)

In the above-mentioned case, the resistivity of the n⁺-layer 3 wasvaried by changing the flow rate of silane, but it is also possible toachieve the same object by changing the flow rate of other raw materialgases or by changing the mixing ratio of silane and other raw materialgases.

In this example, silane was used as the Si dopant, but it can bereplaced by an organosilicon compound such as tetraethylsilane(Si(C₂H₅)₄) in a gaseous state prepared by bubbling with H₂.

The process mentioned above permits one to prepare the n⁺-layer 3 ofhigh carrier concentration and the n-layer 4 of low carrierconcentration in such a manner that their resistivity can be controlledas desired.

The thus obtained light-emitting diode 10 was found to have a luminousintensity of 0.2 mcd. This value is 4 times higher than that of theconventional light-emitting diode which is composed simply of an i-layerand an n-layer. In addition, the inspection of the luminescent surfacerevealed that the number of luminescent points is much greater than thatof the conventional light-emitting diode.

What is claimed is:
 1. A semiconductor device comprising: an n-typelayer of a gallium nitride compound that is doped with silicon as adonor at a predetermined doping rate and that has a carrierconcentration controlled at a desired value in a range from 6×10¹⁶/cm³to 3×10¹⁸/cm³.
 2. A semiconductor device as claimed in claim 1, saidsemiconductor device is a light-emitting device.
 3. A semiconductordevice as claimed in claim 1, further comprising a sapphire substrateover which said n-type layer is formed.
 4. A semiconductor device asclaimed in claim 3, further comprising a buffer layer formed betweensaid sapphire substrate and said n-type layer.
 5. A semiconductor deviceas claimed in claim 4, wherein said buffer layer is formed by anorganometallic compound vapor phase epitaxy at a temperature lower thana temperature at which said n-type layer is formed.
 6. A semiconductordevice as claimed in claim 5, wherein said buffer layer is formed attemperature of about 400° C.
 7. A semiconductor device as claimed inclaim 1, wherein said n-type layer is formed by organometallic compoundvapor phase epitaxy.
 8. A semiconductor device as claimed in claim 1,wherein said n-type layer comprises a gallium nitride compoundsemiconductor satisfy the formula Al_(x)Ga_(1−x)N, 0≦×≦1.
 9. Asemiconductor device as claimed in claim 1, wherein said n-type layercomprises gallium nitride.
 10. A semiconductor device as claimed inclaim 11, wherein the mobility of said n-type layer is between 210cm²/Vsec and 320 cm²/Vsec.
 11. A semiconductor device as claimed inclaim 1, which comprises two electrodes formed on a same side of then-type layer, one of said two electrodes being formed so as to makeelectrical contact with said n-type layer.
 12. A semiconductor devicecomprising: an n-type gallium nitride compound that is doped withsilicon at a donor at a predetermined doping rate and that has a carrierconcentration ranging from 1×10¹⁴/cm³ to 1×10¹⁹/cm³.
 13. A semiconductordevice according to claim 12, wherein said semiconductor devicecomprises a light-emitting device.
 14. A semiconductor device accordingto claim 12, further comprising: a sapphire substrate over which saidn-type gallium nitride-based compound is formed.
 15. A semiconductordevice as claimed in claim 14, further comprising: a buffer layer formedbetween said sapphire substrate and said n-type gallium nitride-basedcompound.
 16. A semiconductor device as claimed in claim 15, whereinsaid buffer layer is formed by an organometallic compound vapor phaseepitaxy at a temperature lower than a temperature at which said typegallium nitride-based compound is formed.
 17. A semiconductor device asclaimed in claim 15, wherein said buffer layer is formed at atemperature of about 400° C.
 18. A semiconductor device as claimed inclaim 12, wherein said n-type gallium nitride-based compound is formedby organometallic compound vapor phase epitaxy.
 19. A semiconductordevice as claimed in claim 12, wherein said n-type gallium nitride-basedcompound comprises a gallium nitride compound semiconductor satisfyingthe formula Al_(x)Ga_(1−x)N, 0≦x≦1.
 20. A semiconductor device asclaimed in claim 12, wherein said n-type gallium nitride-based compoundcomprises gallium nitride.
 21. A semiconductor device as claimed inclaim 12, further comprising: two electrodes formed on a same side ofthe n-type gallium nitride-based compound, one of said two electrodesbeing formed so as to make electrical contact with said n-type galliumnitride-based compound.
 22. A semiconductor device comprising: an n-typelayer of gallium nitride-based compound that is doped with silicon as adonor at a predetermined doping rate and that has a carrierconcentration controlled at a desired value in a range from 1×10¹⁴/cm³to 1×10¹⁹/cm³.
 23. A semiconductor device according to claim 22, whereinsaid semiconductor device comprises a light-emitting device.
 24. Asemiconductor device according to claim 22, further comprising: asapphire substrate over which said n-type layer is formed.
 25. Asemiconductor device as claimed in claim 24, further comprising: abuffer layer formed between said sapphire substrate and said n-typelayer.
 26. A semiconductor device as claimed in claim 25, wherein saidbuffer layer is formed by an organometallic compound vapor phase epitaxyat a temperature lower than a temperature at which said n-type layer isformed.
 27. A semiconductor device as claimed in claim 25, wherein saidbuffer layer is formed at a temperature of about 400° C.
 28. Asemiconductor device as claimed in claim 22, wherein said n-type layeris formed by organometallic compound vapor phase epitaxy.
 29. Asemiconductor device as claimed in claim 22, wherein said n-type layercomprises a gallium nitride compound semiconductor satisfying theformula Al_(x)Ga_(1−x)N, 0≦x≦1.
 30. A semiconductor device as claimed inclaim 22, wherein said n-type layer comprises gallium nitride.
 31. Asemiconductor device as claimed in claim 22, further comprising: twoelectrodes formed on a same side of the n-type layer, one of said twoelectrodes being formed so as to make electrical contact with saidn-type layer.
 32. A semiconductor device comprising: an n-type galliumnitride-based compound that is doped with silicon as a donor at apredetermined doping rate and that has a carrier concentrationcontrolled at a desired value in a range not smaller than 6×10¹⁶/cm³.33. A semiconductor device according to claim 32, wherein saidsemiconductor device comprises a light-emitting device.
 34. Asemiconductor device according to claim 32, further comprising: asapphire substrate over which said n-type gallium nitride-based compoundis formed.
 35. A semiconductor device as claimed in claim 34, furthercomprising: a buffer layer formed between said sapphire substrate andsaid n-type gallium nitride-based compound.
 36. A semiconductor deviceas claimed in claim 35, wherein said buffer layer is formed by anorganometallic compound vapor phase epitaxy at a temperature lower thana temperature at which said n-type gallium nitride-based compound isformed.
 37. A semiconductor device as claimed in claim 35, wherein saidbuffer layer is formed at a temperature of about 400° C.
 38. Asemiconductor device as claimed in claim 32, wherein said n-type galliumnitride-based compound is formed by organometallic compound vapor phaseepitaxy.
 39. A semiconductor device as claimed in claim 32, wherein saidn-type gallium nitride-based compound comprises a gallium nitridecompound semiconductor satisfying the formula Al_(x)Ga_(1−x)N, 0≦x≦1.40. A semiconductor device as claimed in claim 32, wherein said n-typegallium nitride-based compound comprises gallium nitride.
 41. Asemiconductor device as claimed in claim 32, further comprising: twoelectrodes formed on a same side of the n-type gallium nitride-basedcompound, one of said two electrodes being formed so as to makeelectrical contact with said n-type gallium nitride-based compound.